The present invention relates to heat exchange systems and more particularly to an improved cooling tower system and associated method for controlling the content and concentration of an aqueous coolant circulated within the cooling tower so that little or no discharge or "blowdown" of the coolant is required during recirculation.
In the field of heat exchange systems, particularly those associated with industry, cooling towers are widely used to remove absorbed heat from a circulating aqueous coolant by evaporating a portion of the coolant before recycling the remainder at lower temperatures for further absorption. The aqueous coolant, whose absorbed heat is drawn from the processing equipment being served, is generally comprised of water and one or more additives, such as corrosion inhibitors to protect the processing equipment, anti-fouling agents to maintain the processing equipment relatively free of scale and sludge deposits, sequestering agents to overcome calcium and iron precipitation, and biocides to prevent biological slime growths. Still another additive is acid, usually sulfuric, which is introduced as required to maintain a desired pH factor in the coolant, typically between about 7.0 and 8.0. Too low pH factor can lead to increased corrosion, whereas too high a pH, in the presence of hard water, can result in scale build-up and other harmful deposits on the heat transfer surfaces of the processing equipment.
In the cooling tower, the warmed aqueous coolant is essentially permitted to flow over a large surface, such as that provided by spaced strips of redwood or plastic, and is there subjected to a forced draft of air to bring about a partial and rapid evaporation of the thus exposed coolant. The remaining coolant, having given up heat energy to supply the heat of vaporization for the evaporated portion, flows to a reservoir from which it is pumped back to the processing equipment for the absorption of more heat, thus completing the standard cycle. As a result of this evaporation occurring within the cooling tower, the remaining coolant has a greater degree of hardness with the concentration of dissolved salts as well as in the precipitation of salts and various suspended solids found in the recirculated coolant. The increased salt concentrations and precipitates, if uncontrolled, can cause serious problems within the heat exchange system, particularly by increasing corrosion and scale build-up on the heat transfer surfaces of the processing equipment that result in inefficient heat transfer. Higher levels of suspended solids in the aqueous coolant are also capable of causing corrosion and other serious problems by forming deposits upon the heat transfer surfaces that, if not regularly cleaned, can result in reduced and frequently uneven heat transfer, poor corrosion inhibitor performance, shortened equipment life and product loss due to ineffectual cooling.
In order to overcome these problems associated with cooling tower evaporation, a procedure has been traditionally employed whereby a certain percentage of the concentrated, remaining coolant is purged from the circulating system, carrying with it a portion of the unwanted scale and deposit-forming inpurities. This procedure, known as bleed-off or blowdown, is generally based on maintaining a materials balance in the system so that the scaling and fouling constituents are not sufficiently concentrated to result in harmful deposits on heat transfer surfaces. Accordingly, the blowdown is usually accompanied by a corresponding replenishment of the amount of discharged coolant by means of raw make-up water having normal concentration levels.
While this technique of blowdown has been relatively successful as a measure for controlling the degree of hardness of the aqueous coolant and for maintaining the dissolved salt and suspended solid levels thereof at acceptable levels, there are certain disadvantages associated with its practice. In the first instance, the amount of the blowdown can represent a considerable loss of water as well as a very significant loss of valuable additives. For example, in a typical, moderately sized cooling tower unit having a rate of circulation of 5,000 gallons per minute (gpm), the total quantity of blowdown over a 24-hour period can amount to 72,000 gallons or more than three times the total content of the system. This discarded water represents a very appreciable loss, both monetarily and as a valuable resource, with replenishment amounting to many thousands of dollars per year.
Apart from these economic costs associated with the loss and replenishment of the coolant, the blowdown also presents attending environmental risks due to the discharge of some very toxic additives, such as widely used chromate inhibitors. Water pollution and waste control thus become added concerns that need to be addressed in cooling tower systems relying on blowdown to control coolant levels. As a result, water treatment systems that require little or no blowdown have become most desirable for use in connection with cooling towers.